1. Field of the Invention
The present invention concerns an imaging system and a method for producing x-ray images and optical images as they are in particular used in the production of medical images.
2. Description of the Prior Art
Imaging systems in which two-dimensional projection images of a subject to be examined are produced with x-rays have been successfully used in a known manner in medical imaging for decades. In spite of modern methods of three-dimensional imaging, such imaging systems are frequently used due to their simple and fast handling and their cost-effective operation. Examples are C-arm apparatuses as they are used in an operating room to produce intra-operative images.
In such a C-arm apparatus an x-ray source and an x-ray detector are arranged at opposite ends of a C-shaped arc. The C-arm is rotated around the patient in the production of an exposure so that a two-dimensional projection image can be produced. Countering the advantage of the simple and cost-effective availability of such images is the disadvantage that such images demand of the user a high degree of abstraction capability, since the user only sees a two-dimensional projection of the organs to be examined. For example, given complicated fractures only a user with good spatial imagination can conclude the actual prevailing relationships from two-dimensional projection images, such that misjudgments are not a rarity.
Additionally, in operative procedures (in which it is important to assess the position of surgical instruments) it is a prevalent practice to produce a number of x-ray images in order to detect the movement of the surgical instrument in the operative field. This is often associated with a high radiation exposure for the patient.
Apparatuses as they are disclosed in U.S. Pat. No. 6,473,489 B2, and U.S. Pat. Nos. 6,447,163 B1, and 6,229,873 B1 and U.S. Pat. No. 6,227,704 B1 provide an assistance for the user. In addition to the two-dimensional projection x-ray image, an optical image of the subject to be examined is produced in such apparatuses. Both images coincide in their viewing angle, meaning that the optical beam path and the beam path of the x-rays correspond with one another so that a reasonable superimposed presentation of both images can be implemented. A user can thereby assess features of the projection x-ray image relative to features of the surface of the subject.
Such apparatuses have proven to be helpful in interventional procedures. For example, given a surgical treatment of a fracture a physician can initially produce an x-ray image of the fracture. This x-ray image is superimposed on the optical image. If the physician moves a surgical instrument, he can for the most part track the movement of the instrument via the optical image (for example via a video sequence). Since the optical image and the x-ray image are shown superimposed, from the optical presentation of the instrument the physician can thus conclude its spatial position in the operative field, such that fewer x-ray images must be produced. New x-ray images must be produced only when the fracture relationships change due to the operation.
However, in spite of the additional optical image the danger of a misjudgment of the actual prevalent relationships also arises here (albeit to a lesser degree), in particular if the orientation of the surgical instrument in the optical image is ambiguous in relation to the x-ray image due to the projection.
A system with which an image of a first subject (for example a patient) can be acquired is known from DE 199 51 502 A1. Furthermore, the position from which the image was produced and the position of a second subject (for example a surgical instrument) can be determined with the system, such that now the second subject can be mixed into the image of the first subject. An exposure of the patient with x-rays can also be reduced via such a system since a movement of the instrument can be tracked via the position determination. However, the use of a position determination system in an operating room is comparably complicated. Moreover, the use of conventional surgical instruments is not possible since these do not interact with the position determination system.